MIL-STD-810G discusses replication of a time trace under Time Waveform Replication (TWR) in Method 525. TWR is performed to:
- Provide a degree of confidence that the materiel can structurally and functionally withstand the measured or analytically specified test time trace(s) to which the materiel is likely to be exposed in the operational field environment.
- Experimentally estimate the materiel’s fragility level in relation to form, level, duration, or repeated
application of the test time trace(s).
Method 525 discusses TWR from a single-exciter/single-axis (SESA) perspective regarding a single-axis vibration table/exciter. Multiple-exciter/multiple-axis analysis is addressed in Method 527. Method 525 is rather lengthy at 22 pages with a 23 page annex and a second 11-page annex.
SESA TWR consists of the replication of either measured or analytically specified time trace(s) in the laboratory with a single exciter in a single direction, and is performed to accurately preserve the spectral and temporal characteristics of the environment.
Method 525 is broadly consistent with the philosophy of test tailoring. A substantial high amplitude field measured time trace has the potential for producing adverse effects on all electronic materiel. The potential for adverse effects increases with decrease in transition time to the time trace and duration of the time trace.
Until recently, the replication of time traces representing measured samples of field environments varying in time and even frequency, or a combination of both time/frequency variations, was not possible using commonly available exciter control system software. The advent of more powerful data processing hardware/software, and the implementation of advanced control strategies, has led to exciter control system hardware and software that permit convenient replication of extended time-varying test environments on a single exciter in a single direction in the laboratory. TWR test methodology strongly reflects the concept of “test tailoring.”
Limitations
This method addresses very general time-varying traces not necessarily identifiable with underlying stationary or nonstationary random processes. It is apparent from various vendor TWR hardware/software configurations that the only requirement for application of Method 525 is the bandlimited character of the time trace for replication, and its compatibility with the bandlimited characteristics of the device (exciter) to be driven with the TWR hardware/software. For example, measured time traces that vary in frequency can be replicated as long as the time trace bandwidth is limited to overall bandwidth of the exciter control system.
Analysis of Results
Analyses of vibration related failures must relate the failure mechanism to the dynamics of the failed item and to the dynamic environment. It is insufficient to determine that something broke due to high cycle fatigue or wear. Include in failure analyses a determination of resonant mode shapes, frequencies, damping values and dynamic strain distributions, in addition to the usual material properties, crack initiation locations, etc.